Organic photoconductive materials are excellent in mass productivity and incur little risk of causing pollution problems when discarded, compared with inorganic photoconductive materials such as selenium alloys, cadmium sulfide and zinc oxide. They have therefore been widely used as photoreceptors for copying machines or printers utilizing electrophotographic systems. In particular, the function separation type laminated photoreceptors are high in material selectivity and easy in functional design, compared with the monolayer type photoreceptors, so that applied research and development thereof in many fields have been made. The function separation type laminated photoreceptors comprises charge generating layers for generating carriers (charge) by light and charge transporting layers for efficiently transporting the carriers generated in the charge generating layers and neutralizing surface charge, and have the possibility of providing electrophotographic photoreceptors high in electrophotographic characteristics such as charging characteristics, sensitivity and residual potential.
Of these, the characteristics of charge transporting materials constituting the charge transporting layers require efficient receiving of the carriers generated in the charge generating layers, rapid transportation of the carriers in the photoreceptor layers and rapid neutralization of surface charge. For example, charge transporting materials such as hydrazone compounds (JP-A-59-223432 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-60-173112), triphenylamine compounds (JP-A-7-173112), distyryl compounds (JP-A-63-269158), triphenylamine dimer compounds (N,N,N',N'-tetraphenylbenzidine compounds) (Denshi Shasin Gakkai Shi (Journal of Electrophotographic Society) 30, 16-21 (1991) and heterocyclic compounds such as poly-N-vinylcarbazole and oxadiazole are already known. When they are used alone, however, some compounds are liable to suffer from light fatigue and cause an increase in residual potential in repeated use. Further, there are still many unsolved problems with respect to the compatibility with binder polymers, and the like.
On the other hand, it has been attempted that specified compounds selected from the known charge transporting materials are used in combination. For example, mixtures of hydrazone compounds and tetraphenylbutadiene compounds (JP-A-63-223755), and mixtures of distyrene compounds and butadiene compounds (JP-A-3-252861) are known. The former discloses that the problem of the light fatigue phenomenon is solved, and the latter discloses that the problem of prevention of a decrease in residual potential is solved. However, even such photoreceptors are not sufficiently high in light sensitivity yet.
Further, use of two positive hole transporting materials, poly(N-vinylcarbazole) and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1-biphenyl)-4,4'-diamine, as a mixture is described in J. Phys. Chem. 88 (20), 4714-4717 (1984). However, it was observed that combined use of the two materials more reduced the mobility at certain mixing ratios than single use of each material. Accordingly, in some cases, even if two kinds of compounds selected as charge transporting materials both independently exhibit high mobility, they result in decreased performance of the charge transporting materials, leading to the failure to sufficiently keep the characteristics necessary for electrophotographic photoreceptors.
As described above, for the charge transporting materials obtained by mixing two kinds of compounds, ones satisfying all various desirable characteristics such as high charging performance in the dark, high static charge keeping ability, high mobility of charge generated in light irradiation and formation ability of charge transporting layers which are high in solubility in binder polymers and homogeneous have not necessarily been obtained yet at present.
Accordingly, in the charge transporting materials, it has been expected to obtain materials in which the above-mentioned disadvantages are overcome, higher carrier mobility can be exhibited, and various excellent characteristics can be expressed even when the electrophotographic photoreceptors are formed, and the electrophotographic photoreceptors comprising the charge transporting materials having such excellent characteristics have been desired.